1. Field of the Invention
The present invention relates to a motor which uses a permanent magnet, and particularly to a motor structure suitable for a linear motor.
2. Description of the Related Art
FIG. 13 is a diagram showing a synchronization type linear motor according to related art. FIGS. 14, 15, 16, and 17 are diagrams showing a linear motor according to related art disclosed by the present applicant in Japanese Patent No. 3,344,645, which provides a solution to certain problems associated with a synchronization type linear motor. FIG. 15 is a diagram showing a cross section AA of FIG. 14 and FIG. 16 is a diagram showing a cross section BB of FIG. 14. FIG. 17 is a diagram showing a mover 3 seen from the side (side view) and from the bottom (bottom view).
Characteristics of a synchronization type linear motor according to the related art will now be described. In FIG. 13, reference numeral 63 represents a mover, reference numeral 67 represents slots provided in the mover 63 and illustrated by S1–S12, and two-pole three-phase AC winding which is typically used in a rotational induction motor is wound in the slot 67. Reference numeral 65 represents a mover salient pole, reference numeral 61 represents a stator, and reference numeral 64 represents permanent magnets provided on a surface of the stator in which the N poles and S poles are alternately placed.
In general, a force F generated by a winding of one turn of the motor can be represented, according to Fleming's law, as F=B·I·L, wherein B is a magnetic flux density, I is a current, and L is an effective length of electrical wire. Similarly, a power P can be represented as P=F·dX/dt, wherein X is a position of the mover in the movement direction and dX/dt is the velocity of the mover.
Electrically, the power P is represented by P=V·I=dφ/dt·I wherein V is voltage and φ is a magnetic flux linked to the winding of one turn. When a change of magnetic energy within the liner motor is disregarded, an equation, P=F·dX/dt=dφ/dt·I can be deduced from the above-described formulae, and a thrust F generated by the linear motor is F=dφ/dX·I. The thrust F generated by the linear motor is therefore directly proportional to a rate of change of the magnetic flux φ linked to the winding with respect to position, dφ/dX.
Therefore, although not shown, when structure of a mover and a stator similar to that shown in FIG. 13 is employed with a two-pole permanent magnet type linear motor, the generated torque T, that is, the rate of change of magnetic flux p linked to the winding with respect to position, dφ/dX, is nearly directly proportional to the magnetic flux density B.
Similarly, if a linear motor as shown in FIG. 13 is configured such that the winding of one turn is wound from the front side of the page toward the backside of the page at the slot S2 and from the backside of the page toward the front side of the page at the slot S8, when the mover 63 moves to the right by a slight amount, ΔX, the rate of change of magnetic flux φ linked to the winding wound from slot S2 to the slot S8 with respect to position, dφ/dX is almost equal to Δφ/ΔX. The slight change in magnetic flux Δφ is thus an increase in N pole magnetic flux corresponding to the slight change in position ΔX in the mover salient pole 65 sandwiched between slots S2 and S8 and a large change of magnetic flux with respect to position, Δφ/ΔX, can be obtained. Therefore, in a simplified expression, the rate of change of magnetic flux with respect to position, Δφ/ΔX, is approximately five to six times that of the two pole permanent magnet type linear motor as described above and the output thrust is also approximately five to six times that of the two pole permanent magnet type linear motor. Such a Vernier type linear motor using a permanent magnet has a characteristic that it fundamentally has a large thrust. However, the driving frequency for controllably driving this example motor is approximately six times that of the two pole permanent magnet type linear motor, and, because of the limit of the driving frequency and inductance of the winding, in general, high speed driving is not possible.
On the other hand, in a synchronization type linear motor of the related art, there is a problem in that the magnetic flux of each permanent magnet 64 cannot be effectively utilized. For example, when the magnetic flux in the mover salient pole between the slots S2 and S3 is considered, the mover salient pole 65 opposes the N pole of the permanent magnet 64 with a small gap therebetween and there is a magnetic flux of N pole on the mover salient pole 65. However, at the same time, the magnetic flux of the S poles on both sides also leaks from non-magnetic sections, such as the space between mover salient poles 65 and, thus, there are many components in which the magnetic flux closes between the N and S poles without going though the stator. This component of the magnetic flux of the N pole which closes with the S pole without going through the stator does not contribute to driving operation. As a result, the magnetic flux of N pole in the mover salient pole 65 between the slots S2 and S3 is not sufficiently utilized. For a similar reason, in the other mover salient poles 65, effective magnetic flux cannot be sufficiently obtained, resulting in a problem in that a motor thrust is reduced, even when the motor current is suitably applied.
Another problem is that although a maximum magnetic flux density of an electromagnetic steel plate of the mover salient pole 65 has a large value of about 1.7 Tesla, a maximum magnetic density of the stator magnetic pole is about 1.0 Tesla, even when a rare earth metal which has a large remaining magnetic flux density is used. Therefore, there is a problem in that, structurally, the magnetic flux density cannot be increased. There is, however, a desire to increase motor torque by increasing a magnetic flux density of each magnetic pole in the stator 61.
Another problem is that when the linear motor has a relatively long driving range, the cost of the linear motor is significantly increased because the amount of permanent magnet mounted in the stator 61 is proportional to the length of the motor driving range. Another problem is that when the linear motor is applied to feed driving of a machine tool or the like, iron powder or the like may be present in the surrounding environment of the linear motor, and therefore, the permanent magnet 64 must be carefully covered to prevent attachment of the iron powder or the like. There is also a problem in that, because the length of the magnet portion is relatively long, the cost of the necessary cover is increased. Next, the linear motor of related art shown in FIGS. 14, 15, 16, and 17 which is disclosed by the present applicant in Japanese Patent No. 3344645 for solving the problems associated with a synchronization type linear motor as described above will be described.
In FIGS. 14 and 17, a mover 3 comprises three pairs of mover magnetic poles 4a, 4b, and 4c each having three pairs of N magnetic poles 5, S magnetic poles 6, and auxiliary magnets 10 between the N magnetic pole 5 and the S magnetic pole 6, which are provided along a direction of movement of the mover 3. AS shown in FIG. 17, the N magnetic pole 5, the S magnetic pole 6, and the auxiliary magnet 10 are placed so that N pole and S pole are alternately provided. As shown in FIGS. 15 and 16, the N magnetic pole 5 and the S magnetic pole 6 respectively comprises structures 5a and 5b and structures 6a and 6b. The structures 5a and 6a correspond, respectively, to the N auxiliary magnetic pole and the S auxiliary magnetic pole described in Japanese Patent No. 3344645.
A three-phase AC winding 16 is wound around each of the mover magnetic poles 5 and 6. U-phase windings are represented in these figures by reference numerals SU1 and SU2, V-phase windings are represented in these figures by reference numerals SV1 and SV2, and W-phase windings are represented in these figures by reference numerals SW1 and SW2. Each of the mover magnetic poles 4a, 4b, and 4c are placed in positions each of which is shifted by an electrical angle of 120 degrees with respect to the salient pole 2 of the stator 1.
Electromagnetic steel plates forming the mover 3 are an N pole magnetic yoke 3a and S pole magnetic yoke 3b in FIGS. 15 and 16 and these structures are respectively magnetically connected to the N magnetic pole 5 and the S magnetic pole 6. For the mover magnetic pole 4a in FIG. 17, a cross section of the S pole magnetic yoke 3b is shown in order to facilitate understanding of the shape of the mover 3.
A common permanent magnet 13 which is magnetically connected to the N pole magnetic yoke 3a and the S pole magnetic yoke 3b is placed in the mover 3.
When a current is applied to the three-phase AC winding 16 in a linear motor of the related art constructed as described above, the three pairs of mover magnetic poles 5 and 6 are excited to either N pole or S pole depending on the application direction of the U-phase, V-phase, and W-phase windings and a large magnetic pole of N pole or S pole is formed. A magnetic flux 19 passing through each of the mover magnetic poles 5 and 6 and the common permanent magnet 13 passes through the side of the stator 1 and a three-dimensional magnetic path is formed. At this point, a magnetic attraction force is generated corresponding to the positions of the mover 3 and the stator 1 and a thrust is output in the mover 3.
The flow of the magnetic flux will now be described in more detail. When a current is applied from the U-phase to the V-phase and W-phase, that is, when a current is applied such that the SU1, SV2, and SW2 become positive and SU2, SV1, and SW1 become negative, the mover magnetic pole 4a in FIG. 14 becomes an S pole and the mover magnetic poles 4b and 4c become N poles. As shown by the magnetic flux 19, a magnetic flux flows from the S pole magnetic yoke 3b on the backside of the mover magnetic pole 4a to the N pole magnetic yoke 3a on the front side of the mover magnetic poles 4b and 4c, and then, from the front side of the N magnetic poles 5 of the mover magnetic poles 4b and 4c to the stator. Finally, the magnetic flux returns from the S magnetic pole 6 of the mover magnetic pole 4a to the S pole magnetic yoke 3b and a three-dimensional magnetic path is formed. In this process, a force acts in a direction shown by the arrow in FIG. 14, at the boundary between the mover 3 and the stator 1, and the mover 3 moves to the right.
As described above, in the linear motor of the related art as shown in FIGS. 14–17, the N magnetic poles 5 and the S magnetic poles 6 provided in the mover magnetic poles 4a, 4b, and 4c become a common magnetic pole when a current is applied to the windings. Unlike the synchronization type linear motor of the other related art, because there is no leak magnetic flux in which a magnetic flux is closed between an N pole and an S pole without going through the stator, the motor thrust is improved.
Moreover, by supplying a magnetic flux from both the common permanent magnet 13 and the auxiliary magnet 10, it is possible to increase the magnetic flux density to approximately 1.7 Teslas which is the saturation magnetic flux of the electromagnetic steel plate, and thus, it is possible to generate a large magnetic flux on the surface of the mover 3 and, consequently, a large thrust.
Furthermore, by providing both the permanent magnet and the windings 16 on the side of the mover 3, it is possible to achieve a simple structure for the stator 1 in which an electromagnetic steel plates are layered instead of the structure of a synchronization type linear motor of the related art in which an expensive permanent magnet is used on the side of the stator 1 having a long stroke. With this structure, it is possible to reduce the cost of the motor, and, because there is no permanent magnet in the stator 1, debris such as chips does not attach, and environmental resistance can be improved.
In the linear motor of the related art described above, however, there are disadvantages, such as those that are next described.
A cross sectional area of entrances 14 and 15 of yokes shown in FIGS. 15 and 16 are reduced to approximately half of the cross sectional area of the boundary between the mover 3 of the N and S magnetic poles 5 and 6 and the stator 1. Because of this, there is a problem in that the magnetic flux is saturated in this area and the motor thrust is reduced.
In addition, as the material of the mover 3 used in the linear motor of the related art, a structure has been employed in which electromagnetic steel plates are layered in directions indicated in the N pole magnetic yoke 3a and the S pole magnetic yoke 3b in FIG. 15, in order to reduce generation of iron loss in a high speed range. As described above, when a current is applied to such a winding 16 of the mover 3 formed using electromagnetic steel plates, the magnetic flux crosses the electromagnetic plates across the direction of layering as shown in FIGS. 14–16 and is generated as a three-dimensional magnetic path within the mover 3 and the stator 1. At this point, when a current applied to the three-phase winding 16 is changed in order to drive the liner motor, the magnetic flux generated across the direction of layering of the electromagnetic steel plates, in particular, near the N and S magnetic poles 5 and 6 of the mover 3, significantly changes. As a result, there remains with this art a problem in that an eddy current flows in the electromagnetic steel plates forming the magnetic poles 5 and 6, iron loss is generated corresponding to the electrical resistance of the electromagnetic steel plates, and an undesirable amount of heat is generated in the motor.
In addition, because the eddy current described above acts against the magnetic flux passing through the magnetic poles 5 and 6, there had been a problem in that the magnetic flux flowing through each of the magnetic poles 5 and 6 is reduced and the motor thrust is reduced. Similarly, because a magnetic flux is generated across the direction in which the electromagnetic steel plates are layered, a non-magnetic insulating coating applied on a surface of each electromagnetic steel plate forming the magnetic poles 5 and 6 and magnetic yokes 3a and 3b of the mover 3 and an air layer between electromagnetic steel plates would act as a magnetic insulating portion, resulting in a problem in that, because the magnetic flux flowing through each of the magnetic poles 5 and 6 is reduced, the output thrust is reduced.
The present invention was conceived to solve the problems described above, and advantageously provides a linear motor in which a motor thrust is improved with a structure which would prevent saturation of magnetic flux around yoke entrances 14 and 15 and heat generation in the motor is reduced and the motor thrust is improved with a structure in which a magnetic flux is generated in a direction perpendicular to the direction of layering of the electromagnetic steel plates at the magnetic poles 5 and 6 of the mover 3.